Problem: If $x \triangleleft y = x(y-4)$ and $x \oplus y = x-4y$, find $-1 \oplus (2 \triangleleft 5)$.
Solution: First, find $2 \triangleleft 5$ $ 2 \triangleleft 5 = 2(5-4)$ $ \hphantom{2 \triangleleft 5} = 2$ Now, find $-1 \oplus 2$ $ -1 \oplus 2 = -1-(4)(2)$ $ \hphantom{-1 \oplus 2} = -9$.